std: refactor test-runner to no longer require tests to be exported (#9355)

# Description
Test runner now performs following actions in order to run tests:
* Module file is opened
* Public function with random name is added to the source code, this
function calls user-specified private function
* Modified module file is saved under random name in $nu.temp-path
* Modified module file is imported in subprocess, injected function is
called by the test runner
# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
* Test functions no longer need to be exported
* test functions no longer need to reside in separate test_ files
* setup and teardown renamed to before-each and after-each respectively
* before-all and after-all functions added that run before all tests in
given module. This matches the behavior of test runners used by other
languages such as JUnit/TestNG or Mocha
# Tests + Formatting

# After Submitting

---------

Co-authored-by: Kamil <skelly37@protonmail.com>
Co-authored-by: amtoine <stevan.antoine@gmail.com>

crates/nu-std/src/lib.rs

@@ -18,6 +18,7 @@ pub fn load_standard_library(
 
         let mut std_files = vec![
             ("mod.nu", include_str!("../std/mod.nu")),
+            ("testing.nu", include_str!("../std/testing.nu")),
             ("dirs.nu", include_str!("../std/dirs.nu")),
             ("dt.nu", include_str!("../std/dt.nu")),
             ("help.nu", include_str!("../std/help.nu")),